Martin P, Hudspeth A J
Howard Hughes Medical Institute and Laboratory of Sensory Neuroscience, The Rockefeller University, 1230 York Avenue, New York, NY 10021-6399, USA.
Proc Natl Acad Sci U S A. 2001 Dec 4;98(25):14386-91. doi: 10.1073/pnas.251530498. Epub 2001 Nov 27.
The auditory system's ability to interpret sounds over a wide range of amplitudes rests on the nonlinear responsiveness of the ear. Whether measured by basilar-membrane vibration, nerve-fiber activity, or perceived loudness, the ear is most sensitive to small signals and grows progressively less responsive as stimulation becomes stronger. Seeking a correlate of this behavior at the level of mechanoelectrical transduction, we examined the responses of hair bundles to direct mechanical stimulation. As reported by the motion of an attached glass fiber, an active hair bundle from the bullfrog's sacculus oscillates spontaneously. Sinusoidal movement of the fiber's base by as little as +/-1 nm, corresponding to the application at the bundle's top of a force of +/-0.3 pN, causes detectable phase-locking of the bundle's oscillations to the stimulus. Although entrainment increases as the stimulus grows, the amplitude of the hair-bundle movement does not rise until phase-locking is nearly complete. A bundle is most sensitive to stimulation at its frequency of spontaneous oscillation. Far from that frequency, the sensitivity of an active hair bundle resembles that of a passive bundle. Over most of its range, an active hair bundle's response grows as the one-third power of the stimulus amplitude; the bundle's sensitivity declines accordingly in proportion to the negative two-thirds power of the excitation. This scaling behavior, also found in the response of the mammalian basilar membrane to sound, signals the operation of an amplificatory process at the brink of an oscillatory instability, a Hopf bifurcation.
听觉系统在很宽的振幅范围内解读声音的能力取决于耳朵的非线性响应。无论是通过基底膜振动、神经纤维活动还是感知到的响度来衡量，耳朵对小信号最为敏感，并且随着刺激变强，其响应逐渐减弱。为了在机械电转导层面寻找这种行为的相关因素，我们研究了毛束对直接机械刺激的反应。如附着的玻璃纤维的运动所示，来自牛蛙球囊的活性毛束会自发振荡。纤维基部仅 +/-1 纳米的正弦运动，相当于在毛束顶部施加 +/-0.3 皮牛的力，会导致毛束振荡与刺激产生可检测到的锁相。虽然随着刺激增强，夹带现象会增加，但直到锁相几乎完成，毛束运动的幅度才会上升。毛束在其自发振荡频率下对刺激最为敏感。远离该频率时，活性毛束的敏感度类似于被动毛束。在其大部分范围内，活性毛束的反应随着刺激幅度的三分之一次幂增长；相应地，毛束的敏感度会按照激发的负三分之二次幂下降。这种缩放行为，在哺乳动物基底膜对声音的反应中也有发现，标志着在振荡不稳定性边缘（霍普夫分岔）的放大过程在起作用。
